Many different systems are proposed for making photocopies. Roughly, there are two different systems, one is the xerography system in which color toner is used, and the other, the imaging system in which a photosensitive sheet having a carrier such as paper, etc. with coloring dyes coated over the carrier is used. According to the xerography system, an image or a pattern is formed on a charged photosensitive member by utilizing a suitable optical system, powder of colored resin called toner is adhered to a not-irradiated portion on the photosensitive member where the charge is still retained, the adhered powder is transferred onto a copying paper, and then the transferred powder forming an image is fixed to obtain a copy. The xerography system described above requires a photoconductor, a toner applying apparatus and a heat fixing device of the toner. Moreover, as the toner is consumed, it is necessary to always store and supplement the toner. For this reason, the construction of the apparatus becomes complicated and the maintenance thereof is troublesome.
In order to solve the above problems, imaging systems based on photosensitive sheets are known. F.W. Sanders et al, U.S. Pat. Nos. 4,399,209 and 4,440,846 describe an imaging system wherein a photosensitive layer comprising microcapsules containing a photosensitive composition in the internal phase is image-wise exposed to actinic radiation and subjected to a uniform rupturing force whereupon the microcapsules rupture and image-wise release the internal phase. Image-forming agents such as substantially colorless chromogenic materials are typically associated with the microcapsules, so that when the microcapsules rupture the chromogenic materials are able to image-wise react with a developer material and produce a color image. This imaging system provides an apparatus of simpler structure.
However, since such a photosensitive sheet has a low sensitivity and requires a powerful light source for forming a latent image thereon, the light irradiating an original image has caused an excessive heat build-up at the original image support station.
Furthermore, since such photosensitive sheet has a characteristic in which the change of the optical density with respect to the increase of incident light energy steeply occurs, it is difficult to obtain a precise tone gradation.